Natural killer (NK) cells provide rapid early responses to viral infections and thus can contribute substantially to disease modulation and vaccine protection. Traditionally, NK cells have been considered to be nonspecific components of innate immunity, but recent studies in mice have shown that NK cells can also demonstrate features of antigen-specific memory. Until recently it was unclear whether this phenomenon might exist in primates, but our preliminary data demonstrates for the first time evidence of antigen-specific NK cell memory in any primate species. As compared with NK cells from uninfected macaques, splenic and hepatic NK cells from simian immunodeficiency virus (SIV)- and cytomegalovirus (CMV)-infected animals were highly reactive to Gag-, Env-, and pp65-pulsed dendritic cells (DCs), respectively, but nonreactive to control proteins. Interestingly, antigen-specific responses were generally undetectable in peripheral blood, suggesting memory NK cells may be tissue-restricted. The longevity, functionality, and specificity of memory NK cell responses in primates suggest their functional relevance and their potential importance in controlling CMV and HIV. In this new proposal we will address major deficits to this new field of study including identifying distribution, breadth, phenotypes, and functions of NK cell memory responses using state-of-the-art techniques. We will evaluate the overarching hypothesis that memory NK cell responses elicited against SIV and rhCMV are broad, restricted to sites of virus replication or secondary lymphoid organs, and have unique phenotypic and functional properties. We will evaluate this hypothesis with two focused Specific Aims: (1) Evaluate localization and breadth of anti- rhCMV and anti-SIV NK memory responses in tissues; and (2) Identify and quantify NK cell memory subpopulations associated with antigen-specific recognition in rhCMV- and SIV-infected rhesus macaques.